Biodegradable synthetic resin sheet material containing starch and a fatty material

ABSTRACT

A biodegradable composition comprises a synthetic resin, a biodegradable granular filler such as natural starch and preferably a substance autoxidizable to yield a peroxide which attacks the carbon to carbon linkages in the resin.

This application is a continuation-in-part of Ser. No. 353,414 filedApr. 23, 1973 and now abandoned.

The present invention relates to a biodegradable product based on asynthetic resin, which composition combines the physical and chemicalproperties of the resin on which it is based with biodegradability.Biodegradability is the break down, ultimately to complete destructionas a result of the action of living micro-organisms such as bacteria andfungi and the enzymes which are substances intermediate in theirmetabolic processes. Biodegradation thus occurs in environments wheresuch micro-organisms are present. Typically biodegradation occurs in anobject which is in contact with soil, buried or partially buried.

Our Application Ser. No. 353,414 (now abandoned) of which thisapplication is a continuation-in-part proposes a biodegradable productincorporating biodegradable particles constituted by natural starchgranules. Such particles as well as being biodegradable should havesmall relatively regular granules, be heat stable and have relativelyrestricted water solubility in the natural state. However a moistenvironment is the most favourable for biological attack. Naturallyoccurring starch granules as set out in Application Ser. No. 353,414 arethe most appropriate biodegradable material but the present inventiondoes not preclude the use of other materials having the properties setout above.

Thus the principal objective of the present invention is to provide abiodegradable product which combines the desirable properties ofplastics i.e. strength, ease of forming into desired shapes andresistance to chemical attack with significant biodegradability suchthat a product buried in the soil will degrade to destruction typicallyin 12 months. Thus products in accordance with the invention will make asignificant contribution to the environment by reducing the disposalproblem caused by chemically inert and bulk plastics.

A possible alternative to starch is a lactose sugar.

Our experimental observations suggest that although a product inaccordance with our aforementioned earlier application constitutingessentially of the relevant resin and the biodegradable filler willdegrade in a satisfactory manner in municipal garbage disposaloperations the biodegradation has not been so effective in natural soil.The present invention seeks not only to improve the biodegradability ofthe plastics-based material in municipal garbage disposal operations butalso in natural conditions.

Starch-filler products as envisaged above achieve their biodegradabilityessentially by the filler particles exposed i.e. at or adjacent thesurface initially being degraded and leached away followed successivelyby the degradation of particles at the interior to produce a cellularstructure which is more readily attacked by the processes of oxidation,hydrolysis, direct enzyme action or combinations of these processes. Thepresent invention seeks to achieve biological attack of the large stablemolecules of the plastics. The characteristic that virtually allplastics share is a large number of carbon to carbon linkages. It isthis structure which produces the desirable properties of the plastics.

The present invention provides a biodegradable product including asynthetic resin, a biodegradable filler and a material which in anatural environment oxidizes itself (autoxidises) to generate a peroxidewhich peroxide attacks the carbon to carbon linkages. Such a materialshould be physically and chemically compatible with the resin andcommercially convenient and readily available materials which fullfillthese conditions are the derivatives of unsaturated fatty acids whichcontain at least one double bond per molecule such a their alkyl estersor the natural fats themselves. The effectiveness of fats to achievebiodegradation is reinforced by our experience in municipal garbagedisposal operations wherein there is a relatively high percentage ofextractable oily matter.

The autoxidation of compounds such as fats can be catalysed bytransition metals such as iron. In the soil burial situation such metalsare available. We believe that the consequences of soil burial arefirstly a reaction between the transition metal salts generally presentin soil with fatty acid at the interface between the plastics-basedcomposition and the soil, secondly a migration of the fat solubletransition metal salt of the fatty acid into the composition andthirdly, autoxidation of the salt thereby generating the peroxidesand/or hydroperoxides which initiate the break down of the plasticsmolecules. Thus preferably the composition in the present inventionincludes as well as the fat or other autoxidisable substance a fattyacid to initiate the release of the iron salts for the autoxidation. Thefat soluble transition metal salts can be preformed and included in theplastics based composition in accordance with the invention. Howevereven without the specific inclusion of a fatty acid rancidity shouldgenerate fatty acid from a fat. The present invention is particularlyapplicable to thermoplastic resins such as flexible polyethylene andpolypropylene packaging sheet which give rise to the greatest disposableproblems. However the present invention is applicable to all plasticswhich include carbon to carbon linkages. Compositions based onpolystyrene, polyvinyl chloride, and polyurethane are specificallyenvisaged.

Natural starch granules are the preferred biodegradable additives incompositions according to the invention. Starches are polysaccharidecompounds which on hydrolysis produce sugars. More accurate definitionsand discussions of the properties of starches can be found in the textbook "Starch Chemistry and Technology" by Whistler-Pachall published bythe Academic Press and in the book "The Starch Industry" by J. W. Knightpublished by the Pergamon Press. Multi-occupation starches such as riceand maize starches are preferred and, in order to optimise the starchconcentration in a composition according to the invention, a mixture oftwo starches each having a different grain size is preferred. Thus, amixture of rice and maize starch is most suitable. A multi-occupationstarch is a starch in which more than one granule occupies the cell inwhich the starch is created. Such starches have granules of polyhedralform. In contrast single occupation starches are created as singlegranules, each in a cell, and have a smooth ovoid granule. The greaterthe starch concentration in the composition the greater thebiodegradability. These properties increase progressively from a minimumof say 5% and we have found that a 15% starch/85% resin composition issatisfactory though higher starch concentrations up to say 50% canadvantageously be employed in certain circumstances. Generally howeverwith a starch concentration of over 30% by weight the properties of theproduct will start to differ significantly from the plastics on whichthe composition is based.

In thermoplastic-resin-based compositions the starch granules willnormally be mixed with the raw polymer during processing to prepare theraw polymer for moulding, extrusion or other fabrication procedure. Suchprocedures take place under relatively severe physical conditions i.e.at certain stages the composition is subjected to elevated temperaturesand pressure. It has been found that natural starch granules survivethese procedures. However if the polymerisation takes place underconditions such that the starch is not altered chemically or physicallythen the starch granules can be added to the monomer. An example of sucha polymerisation is the production of polystyrene. As the polymerisationof the elastomers is a thermosetting process the starch must be addedbefore polymerisation is complete.

As previously mentioned the present invention has an importantapplication to the flexible packaging materials which are derived fromthermo-plastic resins such as PVC, polyethylene and polypropyleneresins. Such materials are flaccid, do not readily receive printedimpressions, cannot readily be bonded by cold adhesives and cannot behandled by the transport mechanisms of paper-processing machines.Compositions in accordance with the present invention which includestarch have, to some extent at least, the physical characteristics ofpaper i.e. the ability to hold a crease and to receive a printedimpression, to be bonded using normal cold adhesive and to besuccessfully handled in paper-processing machinery. Sheets of thecomposition, moreover, have a reduced tendency to "block" i.e. to sticktogether when pressed together in machine handling operations.

In all the compositions in accordance with the invention treatment withhot water to gelatinize or dissolve surface starch granules renders thecomposition more paper-like.

With regard to the choice of fats and fatty acids for generating theperoxides or hydroperoxides by autoxidation the reaction mechanismdepends on the presence of unsaturated linkages in the molecularstructure of the fatty acids. Thus each molecule should contain at leastone double bond and the commonest example of such a mono-olefinicunsaturated fatty acid is oleic acid. Where a greater speed of reactionis required fatty acids with double or multiple double bands, such aslinoleic or linolenic acid should be selected.

One of the properties of starch is its "chemisorbtion" i.e. its abilityto adsorb onto its surface chemicals thus the present inventionenvisages chemisorbing the fatty acids onto the surface of the starchwhich is then added to a composition, already including the fatty acidester. Because fats i.e. the esters of the fatty acids are morephysically compatible with the thermoplastics than free fatty acids itis advantageous to adsorb the fatty acids onto the starch and to add theester separately.

Typically in a composition according to the invention including alsoresin and a biodegradable additive there will be up to 2% by weight ofcombined fatty material i.e. fat and fatty acid. The fatty material willinclude typically 90% by weight fatty acid ester and 10% by weight fattyacid.

It has been found that, in order to avoid blemishes in the finishedproduct that it is essential to immobilise unconfined or free water inthe starch or other additive. Any free water can generate steam atcritical points in the manufacturing operation which results in porosityor reduction in strength by weakening the interface between the starchgrains and the plastics material. In circumstances the intensiveexternal drying required is either uneconomic or impossible, theinvention provides the addition of dessiccating or moisture attractingagents capable of immobilising the unconfined water. Desiccating agentssuch as active silica and active alumina may act in a physical manner bythe adsorbtion of the water into a porous structure. As an alternativedesiccating agents may be employed which combine chemically with thewater and in which case they are selected from classes of substancescapable of reacting irreversibly at the temperature and pressureprevailing. Typical examples of such compounds are inorganic anhydroussalts such as calcium sulphate, alkaline earth oxides such as calciumoxide or water reactive organic compounds such as acid anhydrides, alkylor aryl mono or poly isocyanates, or the reaction products of suchisocyanates with phenols or hydroxylamines such reaction products beingcapable of re-generating the parent isocyanates at temperatures reachedin the manufacturing operations. Particularly preferred compounds arethose which will readily form ethers or esters with hydroxyl groups.Thus, for example, isocyanates or their derivatives and silicones reactwith hydroxyl groups in the starch itself at or near the surface of thegrains thus reducing the hydrophyllic nature of the grain surface andincreasing the strength of the starch/polymer bond. It has been found,specifically, that compositions including a silicone surface treatedstarch have very satisfactory physical strength. The invention providesaccording to another important aspect a starch/resin composition whereinthe starch granules are surface treated with one of these substances.

There are many circumstances in which it is advantageous to provide asealed container in which the interior space is maintained at a lowmoisture content. Typical applications would be the packaging of medicalsupplies or hygroscopic chemicals, photographic film, instruments orfine machinery liable to rust or corrode or explosives. In the lastmentioned application the package can constitute the cartridge case orexplosives-containing tube. It has been customary in such packages toinclude a separate loose dessicant such as a silica gel in a poroussachet. A packaging laminate including an inner film made according tothe present invention obviates the need for the separate sachet as thedessicant in the form of starch granules can be incorporated into thefilm. An outer impervious skin, for example of aluminium foil is thenincorporated.

The following examples further illustrate the invention:

EXAMPLE I Composisition including Polyolefin resin, starch, fatty acidester and fatty acid.

200 grams of maize starch which had been pre-dried to 0.5% moisturecontent was tumble blended with 39 grams of ethyl oleate (prepared fromtechnical grade oleic acid and with an iodine value between 75 and 84and density between 0.869 and 0.874) and 1 gram of oleic acid (of iodinevalue between 85 and 90 and density about 0.891) and 160 grams of lowdensity polyethylene film extrusion grade of density 0.920 and melt flowindex 1. The resultant mixture was hot compounded on a 2 roll milloperating at 140° C at even speeds. The operation took approximately 10minutes at the end of which time, the smooth creamy white compound wasstripped from the mill as a hide, approximately 3 m.m. thick, cooled andcut into cubes in a dicing machine. This composition was then used as amasterbatch in admixture with low density polyethylene of density 0.916and melt flow index 1 in such ratios as to give 8% of starch in thefinal composition.

The blend of masterbatch and unfilled polymer was fed to a 45 m.m.single screw extruder with a screw of compression ratio 2.5 : 1 and L:Dratio of 20 : 1 and converted to blown film using conventional die andbubble haul off equipment with the final die temperature of 175° C. Theproduct was a translucent flexible film having a falling dart impactstrength of 220 grams, a tensile strength in the machine direction of7.59 MN/m², and tensile strength in the transverse direction of 6.9MN/m², a tear strength in the machine direction of 1.73 MN/m², and atear strength in the transverse direction of 1.73 MN/m². These resultscan be compared with figures for film produced in the same run on thesame machinery with the same polymer excluding the starch and otheringredients where the falling dart impact strength was 265 grams, thetensile strength in the machine direction 8.28 MN/m², the tensilestrength in the transverse direction 5.52 MN/m², the tear strength inthe machine direction 2.07 MN/m², the tear strength in the transversedirection 1.73 MN/m².

EXAMPLE II Laboratory indication of the Biodegradability of Compositionof Example I

Several sample squares of film, prepared as described in Example I, ofabout 5 centimeter side and 50 micron thickness were cut and placed sideby side on a glass plate in an incubator. Half of the samples had beencoated with a slurry of London clay. The incubator was maintained at atemperature of about 65° C for a period of one month as being typical oftemperatures attained in the early stages of a composting process. Theincubator was then switched off and the samples left in the dark at aroom temperature of about 15° C for a perod of 6 months at the end ofwhich time the clay coated samples cracked when they were sharply bentby hand whilst the uncoated samples were not perceptibly different fromsamples which had been maintained at room temperature and in the darkfor control purposes.

In a further trial polyethylene film samples made as described inExample I with the starch contents adjusted as listed in the table belowwere placed in a cylindrica aluminium drum of about 250 liter capacity,the drum being situated with its long axis arranged horizontally andprovided with a mechanism which rotated about the long axis very slowly.The drum was charged with about 20 kilograms of fresh screened composttaken from a `Dano` composting plant operating normally in the vicinityof London on a mixture of domestic garbage and dewatered sewage sludge.The drum, which was in effect a miniature composting unit, wasmaintained at a temperature of 35° C ± 2° C by a combination of internalthermostat and an external bank of infra-red lamps. The moisture contentof the charge in the drum was maintained at about 50% and free access ofair assured by adequate ventilation. Samples of plastics film asdescribed were withdrawn at monthly intervals and showed a considerableloss of tensile strength by the end of the second month followed afterone further month by the appearance of numerous cracks which madefurther tensile testing impossible. More detailed results are given inthe table below:

    ______________________________________                                                       Initial      Tensile strength                                  Film sample identity                                                                         tensile strength                                                                           after 65 days                                     ______________________________________                                        Blank, i.e. no starch                                                                        8.55 MN/m.sup.2                                                                            6.65 MN/m.sup.2                                   or fatty acid deriv-                                                          ative.                                                                        Generally as Example I                                                                       8.13 MN/m.sup.2                                                                            6.13 MN/m.sup.2                                   but with 9% ww starch                                                         Generally as Example I                                                                       7.3 MN/m.sup.2                                                                             3.65 MN/m.sup.2                                   but with 23% ww starch                                                        ______________________________________                                    

EXAMPLE III Composition including polystyrene thermoplastic resin,starch, fat and fatty acid.

300 grams of predried tapioca starch was tumble blended with 3000 gramsof crystal polystyrene, 30 grams of pharmaceutical grade ethyle oleateand 3 grams of purified oleic acid. The whole blend was then hotcompounded in a Francis Shaw type K1 internal mixer operating with 50pounds steam pressure in its heating system. After hot compounding forsix minutes the charge was dumped onto an 18 × 9 inch two roll millpreheated to 150° C and reduced to a thin hide which was subsequentlypulverised to granular form and the granules fed to a single screwextruder of LD ratio 25 : 1 and screw diameter 45 mm with a two sectionscrew allowing for venting volatiles from the barrel. The extrudate fromthis extruder was taken from a die yielding sheet material 0.7 mm thickand 250 mm wide which sheet was smooth surfaced and suitable for use inthe thermoforming process used for the manufacture of thin walledpackages and disposable drinking cups. Extrusion of sheet and tube wasreadily possible with compositions of the type described containing upto 30% by weight of starch and, making due allowance for increased meltviscosity, up to 50% by weight of starch could be included incompositions for extrusion or compression moulding.

EXAMPLE IV Laboratory indication of biodegradability of Composition ofExample III.

A starch/polystyrene compound prepared as described in Example III so asto contain 50% by weight of starch was converted by the process ofcompression moulding into test pieces 80 mm long 12.7 mm wide and 1.5 mmthick which were tested according to the method of BS 2782 (1970) Method302/D except that a support separation of 60 mm was used with a testingspeed of 12 mm/minute. The samples as prepared failed at a maximum fibrestress of 46.2(3) MN/m² and had an elastic modulus of 2,900 MN/m², butafter immersion for ten days in 0.1% alpha-amylase solution the strengthhad dropped to 21.8(9) MN/m² and the elastic modulus was down to 68.5MN/m² indicating that the filled plastic was progressively attacked bythe enzyme alpha amylase which is known to be produced by common soilmicro-organisms, and the effect could also be followed visually byinspecting the broken surfaces of the test specimens.

EXAMPLE V Composition containing polyethylene, lactose, fat and fattyacid.

1,000 grams of granules of a low density polyethylene of Melt Flow Index2 and density 0.918 were tumble blended with 200 grams of finelypowdered dry lactose. 10 grams of pharmaceutical grade ethyl oleate, and1 gram of purified oleic acid. The resulting blend was hot compounded ona laboratory 2-roll mixing mill preheated to 135° C, giving a smoothwhite hide which was stripped from the mill and chopped into granules.The granulated compound was then hot pressed between polished steelplates to yield a smooth supple film about 0.25 mm thick.

EXAMPLE VI Laboratory indication of biodegradability of Composition ofExample V.

Samples of film made as described in Example V were placed in thelaboratory rotating compost unit as described in Example II, after 70days the samples displayed cracks and brittleness along withdiscolouration indicative of colenisation by bacterial growth.

EXAMPLE VII Composition containing polyolefin, starch, fatty acid esterand desiccant.

A composition was prepared as described in Example I except that theoleic acid was omitted and the starch used was a maize starch of about2% moisture content and the starch concentration was 9% by weight.Attempts to extrude thin blown film from this composition failed becausethe product was disfigured and weakened by the presence of numeroussmall bubbles created by the conversion of the free moisture to steam atthe moment of issue of the plastics melt from the mouth of the extrusiondie. When the same composition was extruded after the addition of asufficient amount of a master batch compounded separately from equalweights of 5 micron mean diameter calcium oxide powder and low densitypolyethylene such as to ensure the presence of one percent by weight ofcalcium oxide in the final extruded composition then the extrusionbecame perfect in that it was free from bubbles. It was necessary toomit the oleic acid to avoid reaction with the calcium oxide.

EXAMPLE VIII Composition (as Example I) including polyolefin, starch,fatty acid ester, fatty acid, but with the starch modified by siliconetreatment.

1200 grams of maize starch of about 12% moisture content was suspendedin 4 liters of water with continuous agitation and to this suspensionwas added 43.2 grams of a 49% ww solution of a sodium alkyl siliconatesold as a proprietary composition by the Dow Corning Company as `DC772`. The pH of the starch slurry was then adjusted to 8.5 by theaddition of dilute acid and the slurry then dried by feeding it to asmall spray drying plant of conventional design working with an airinlet temperature of 190° C and an air outlet temperature of 60 to 70°C., the powdery product being kept in circulated hot air oven at 80° Cuntil its moisture content was 1% or less. The resultant highly waterrepellent starch was used to produce a starch/low density polyethylenemaster batch exactly as described in Example I and this master batch inturn used to make extrusion-blown film. Polyethylene film produced inthe manner described in Example I and containing 9% ww starch in apolymer of Melt Flow Index 2 and density 0.918 grams per cubiccentimeter showed on testing a tensile strength of 14.4 MN/m² and afalling dart impact strength of 70 grams at a film thickness of 50microns as measured by a screw micrometer. When a film containing 9% wwof silicone treated starch was tested under identical conditions thetensile strength was found to be 15.2 MN/m² and the falling dart impactstrength 80 grams. This increase in tensile strength is manifest inresin/starch compositions irrespective of whether the autoxidisablesubstance is present.

EXAMPLE IX Dessicating Packaging Film

An aluminium foil laminate was prepared in a conventional manner byextruding a coating of low density polyethylene of MFI 7 and density0.917 grams per cubic centimeter between webs of bleached kraft paper of50 gsm substance and aluminium foil, this laminate being then passedagain through the extrusion coating plant the outer aluminium surfacewas extrusion coated with a layer of starch/low density polyethylenecomposition formulated as described in Example I except that the starchcontent was adjusted to 30% ww. The quadruple layerpaper/polyethylene/aluminium/starch-polyethylene laminate was convertedinto flat packets, or sachets, approximately 100 mms square by theprocess of heat sealing together the starch containing layers. It isevident by calculation that the inner lining of these sachets contains0.3 grams of starch and this starch has had its moisture content reducedin manufacture to less than one percent. Taking 15° C and 60% RelativeHumidity as being very common conditions for the atmosphere we know thatsuch air will contain 7.92 grams of water per cubic meter and thereforethe walls of the sachets as described are capable of drying 379 cubiccentimeters of air for every 1% increase in the water content of starch.Experimental information is available (Hellman, Boesch, & Melvin,J.A.C.S. Volume 74, P.348) which establishes that even after taking up4.53% of moisture at 25° C the atmosphere in equilibrium with maizestarch has only 8% relative humidity, and tests conducted by sealingdeliquescent or moisture sensitive materials in the sachets made asdescribed in this example indicate that the considerable reserve ofdrying power in the inner coating has reduced the water content of thesmall volume of free air sealed inside to a very low level.

EXAMPLE X Long Term tests of Biodegradability

Multiple samples of sheet materials prepared in accordance with theforegoing examples were buried on the 20th day of November 1974 2 cmsbelow the surface of three types of burial material specified below intrays maintained at approximately 50% moisture content by regularwatering held in a conventional horticultural greenhouse set aside forexperimental work, regular log of soil temperature being kept. The threetypes of burial material were:

1. garden loam recovered from land, originally agricultural, in theCounty of Middlesex, England, screened through a coarse sieve to removestones larger than about 6 mm diameter,

2. Beech forest, surface soil obtained from Burnham Beeches,Buckinghamshire, England, and screened through a coarse sieve to excludeparticles larger than about 6 mm in diameter,

3. screened compost made in a `Dano` continuous composting plant workingon a blend of municipal garbage and de-watered sewage, taken fresh fromthe discharge screen of the plant and adjusted to correspond to theaverage composition of raw compost before entering the plant accordingto anaylsis quote on page 501 of `Handbook of Environmental Control`Volume II Ed. by R. G. Bond and published by CRC Press, Cleveland, Ohio,U.S.A. by additions of sugar, starch, and vegetable cooking oil.

All the samples were carefully characterised physically before burialand corresponding control samples maintained in sealed dry containers inthe dark it being proposed to unearth the buried samples one year fromthe date of internment and repeat the physical characterisation on bothsets with a view to making the results public in an appropriate manner.Control samples of unmodified resins were also buried.

The following further two examples illustrate biodegradable compositionsincluding resin and starch granules.

EXAMPLE XI PREPARATION OF A THERMOPLASTIC-RESIN-BASED COMPOSITION

A masterbatch was prepared by blending together on a laboratory 2-rollmill heated by steam at 140 psig pressure equal weights of low densitypolyethylene and a starch blend. The polyethylene used as AlkatheneQ1388 made by I.C.I. Ltd., which has a density of 0.920 g/ml at 23degrees centrigrade and a M.F.I. of 2 when measured according to method105C of B.S. 2782, and the starch blend comprised equal parts by weightof maize starch and rice starch. The masterbatch material as cut fromthe mill was reduced to granule form in a dicing machine and blendedcold with granules of the unmodified polyethylene and fed to an extruderset up for the manufacture of layflat blown polyethylene film using atypical die and pinch rolls and haul-off equipment of conventional form.The starch content of the resulting blown film was adjusted to variousconcentrations between 5 and 15%, the effect of the added starches beingmore evident at the higher concentrations in that the film showed nosigns of "blocking", was slightly more hazy to visual inspection, andhad a feel and crease retention much more paper-like that the unmodifiedpolyethylene film at similar thickness. Blocking is the local stickingtogether of films pressed together, particularly in the nip of a pair ofrollers.

EXAMPLE XII PREPARATION OF A THERMOSETTING-ELASTOMER-BASED COMPOSITION

200 grams of Daltorol PR₁, a low molecular weight polyester resinmanufactured by I.C.I. Ltd, and 200 grams of a mixture of equal parts ofrefined maize starch and rice starch, were blended together bymechanical stirring in a glass vessel under a reduced pressure of 30mmHg. The temperature of the mixture was raised to 105 degrees centigradeand maintained at that level for approximately 30 minutes in order toremove free moisture from the ingredients. After reducing thetemperature to 70 degrees Centigrade 20 grams of Suprasec SFN, a blendof 2,4- and 2,6-tolylene diisocyanate manufactured by I.C.I. Ltd., wasadded and stirring continued for 2 minutes at atmospheric pressurefollowed by 10 minutes under vacuum as before in order to removeentrapped air during which operations the temperature of the mixremained between 70 and 75 degrees Centigrade largely due to theevolution of heat by the chemical reaction taking place between thepolyester and the di-isocyanate. At the conclusion of the mixing andde-aerating process, the syrupy product was poured onto plane horizontalsurfaces which had been precoated with a suitable release agent, usuallypolytetrafluorethylene, and finally oven cured at 110 degrees centigradefor 3 hours. In a variation of the procedure the syrupy composition wasspread with a steel blade onto the surface of 71 g/m² white bond paperand then subjected to the same oven cure. The unsupported starch filledrubber samples were smooth and supple and substantially free frombubbles or agglomerates of filler, the addition of the starch made nosignificant difference to the curing reaction or time although the tearstrength of the finished rubber, as judged manually, was rather low. Thecoated paper samples were completely permeated by thestarch/polyurethane rubber because of the low viscosity of the preparedsyrup and the resultant composite was strong and impermeable to waterand gases. A further interesting advantage of using starch granules as afiller in polyurethane based compositions is the relatively small lossin strength that is involved. This is illustrated in the followingfurther examples:

EXAMPLE XIII COMPOSITION INCLUDING POLYURETHANE

Equal weight of a thermoplastic polyurethane resin manufactured by theBayer Chemical Co. Under the typecode of Desmopan 385 and pre-driedmaize starch were hot-mixed together on a laboratory 2-roll mill steamheated at approximately 100 psi steam pressure. The sheet of materialstripped from the mill was cut into small granules and blended withfurther Desmopan 385 so as to give a final starch concentration of 31%by weight and this blend was then converted by the process of extrusionblowing into sheet material 0.15 mm thick the product being smooth andvery soft. The tensile strength of this product measured following theprocedure of ASTM D 638-64T, was 16.0 MN/m² as compared with 18.5 MN/m²tensile strength measured on the thermoplastic polyurethane withoutstarch. The loss in strength caused by starch addition being only 13.5%and thus surprisingly low for such a high concentration of filler, byway of comparison a composition of PVC and plasticiser of similarsoftness filled to the same degree showed a strength loss of 49.4 %.

EXAMPLE XIV LABORATORY INDICATION OF BIODEGRADABILITY OF COMPOSITION OFEXAMPLE XIII

Sample of sheet thermoplastic polyurethane containing 31% maize starchprepared as described in Example XIII were partly buried in boxes ofgarden soil and kept at 20° C in a humid atmosphere. After 2-3 weeks anetwork of fungal hypha could be seen at the junction of the soil andthe emerging samples and after 6 weeks the protruding ends fell onto thesail surface having broken away from the buried portions at the time itwas not possible to recover the buried samples other than in fungusriddled fragments.

I claim:
 1. A biodegradable synthetic polymeric composition consistingessentially of a polymer having carbon to carbon linkages and dispersedtherein from 5 to 50% by weight particles of a biodegradable substanceand up to 5.5% by weight of an auto-oxidizable substance containing atleast one double bond which when in contact with a transition metal saltauto-oxidizes to generate a peroxide or a hydroperoxide, said substancebeing selected from the group consisting of a fatty acid, a fatty acidester, a natural fat, and mixtures thereof, whereby said polymericcomposition when disposed in soil containing a transition metal saltwill be biodegraded at a rate faster than a similar product free fromsaid substance.
 2. The composition of claim 1 wherein the biodegradableparticles are natural starch granules.
 3. The composition of claim 1wherein the auto-oxidizable substance is 90% by weight fatty acid esterand 10% by weight fatty acid.
 4. A composition according to claim 1wherein at least part of the auto-oxidizable substance is a fatty acidand is chemisorbed onto the surfaces of the starch granules.
 5. Thecomposition of claim 1 wherein the auto-oxidizable substance contains anatural fat.
 6. The composition of claim 1 wherein the biodegradableparticles are lactose sugar granules.